1. Technical Field
The present invention relates to toner and a development agent, an image forming apparatus, and a process cartridge that use the toner.
2. Background Art
Latent images formed electrically or magnetically are typically rendered visible by an electrophotographic image forming apparatus using toner (electrophotographic toner). For example, in electrophotography, electrostatic images (latent images) are formed on an image bearing member (typically a photoreceptor) and developed with toner to form visible toner images. The toner image is then transferred onto a transfer medium, typically paper, and thereafter fixed thereon. In the process in which the toner image is fixed on the transfer medium, a thermal fixing device such as a heating roller fixing system or a heating belt fixing system is generally used for better energy efficiency.
In recent years, demand for ever faster, more energy-efficient image forming apparatuses has continued to grow. Toner having excellent low-temperature fixability and providing quality images is one of the keys to satisfying such demand.
To attain toner having excellent low-temperature fixing, binder resins forming the toner are required to have low softening temperatures. However, when the softening temperature of the binder resin is low, part of the toner image easily adheres to the surface of the fixing device when fixing the image and is transferred onto the transfer medium (so-called offset, also referred to as hot offset).
In addition, the ability of the toner to withstand high temperatures also deteriorates, leading to clumping (in which the toner particles stick to each other) under high-temperature conditions in particular. Furthermore, there are other problems, such that the toner particles adhere to the inside of a development device or to carrier particles, thereby contaminating the development device or causing toner filming on the surface of the image bearing member.
To solve these problems, use of crystalline resins as the binder resins for toner is known.
Crystalline resins quickly soften at their melting points so that it is possible to lower the softening point of the toner to around its melting point while securing excellent high-temperature stability at the melting point or temperatures lower than that. Therefore, such toner can have a good combination of low-temperature fixing and high-temperature stability.
For example, JP-H04-24702-B (JP-S62-070859-A) and JP-H04-24703-B (JP-S62-070860-A) disclose toners using crystalline resins elongated from a crystalline polyester by diisocyanate as the binder resins.
These toners have excellent low-temperature fixing properties but insufficient hot offset resistance, which is not satisfactory in terms of the level of quality currently required.
In addition, JP-3910338-B1 (JP-2001-305796-A) discloses toner that uses a crystalline resin having a cross-linked structure formed by unsaturated linking containing a sulfonic acid group.
This toner can overcome hot offset in comparison with conventional toner. Further, JP-2010-77419-A discloses regulating the ratio of the softening point to the peak temperature of the melting heat and viscoelasticity to obtain an excellent combination of low-temperature fixability and high-temperature stability.
However, these toners having crystalline resins as the main component of their binder resins, although they have excellent impact resistance, also exhibit poor indentation hardness (e.g., Vickers hardness). As a consequence, due to the stirring stress in a development device, carrier and a machine are easily contaminated and toner filming on a photoreceptor tends to occur. Also, the chargeability and fluidity tend to deteriorate because of burial of an external additive.
In addition, recrystallization of toner melted on a fixing medium during heat fixing takes a time, thereby delaying recovery of the hardness of the image surface.
For this reason, a mark of the discharging roller used in the sheet discharging conducted after fixing is left on the image surface, which causes a change of the gloss or damage.
Moreover, even if the hardness of the image surface is restored by the recrystallization of the toner, the hardness is not sufficient, so that the image is still vulnerable to scratch or abrasion.
JP-3360527-B1 (JP-H09-329917-A) discloses improvement of the stress resistance of toner by regulating the durometer hardness of a crystalline resin in the toner and containing inorganic particulates in the toner. Although successful in some degree, it does not have an impact on a mark of the roller left after fixing or improve the image hardness sufficiently. In addition, the low temperature fixability is worsened by the inorganic particulates. Consequently, this improvement does not stretch the advantage of the fixability of the crystalline resin to the full.
Unlike the above-mentioned, for example, JP-3949526-B1 (JP-2004-038115-A) and JP-4513627-B1 (JP-2006-276305-A) disclose a combinational use of a crystalline resin and a non-crystalline resin instead of using a crystalline resin as the main component.
These toners compensate the defect about the hardness of crystalline resins by the non-crystalline resin but are not capable of exhibiting the power of the crystalline resin for the low temperature fixability improvement to the full.